The present invention relates to a downhole tool capable of forming part of a selectively releasable joint, a downhole drilling assembly that includes that selectively releasable joint and to a method of selectively releasing a part of a downhole drilling assembly from the remainder of the assembly. In particular, the present invention relates to such a tool, assembly and method where a selectively releasable joint is provided which may be released downhole to allow, for example, a drill bit of a drilling assembly to be released from the remainder of the assembly, in the event, for example, that the drill bit becomes stuck during a drilling operation.
In the art of drilling holes in the earth for the purposes of recovering oil and gas accumulations, it is common to use a hydraulic motor to drive the drill bit. In a typical set up a drill bit with a suitable cutting structure is connected to a bottom hole assemblage of drill collars and pipes connected to the surface. The pipes provide a conduit through which a fluid is transmitted to provide hydraulic pressure and flow to the motor. The resultant rotation of the drill bit creates a means for destruction of rock and deepening of the earth bore. In the process of drilling these earth bores it is sometimes possible that the drilling bit becomes stuck in the well bore, for example, due to movements of the rock or other means, thus preventing further deepening of the borehole or preventing extraction of the drilling assembly from the borehole. Under these circumstances it is often necessary to release the drill pipe above the drilling motor and/or any in hole measurement tools, before abandoning or sidetracking the wellbore. This can lead to considerable expense due to the value of the lost equipment and the costs incurred in drilling and recovering the lost wellbore.
The present invention now obviates or at least mitigates at least one of the foregoing disadvantages.